This invention relates generally to filtering a process water stream, and more particularly to filtering a gasification process water stream.
At least some known combined cycle power systems used for power generation include a gasification system that is integrated with at least one power-producing turbine system. For example, known gasifiers convert a mixture of fuel, air or oxygen, steam, and/or limestone into an output of partially combusted gas, sometimes referred to as “syngas.” Hot combustion gases are supplied to the combustor of a gas turbine engine, which powers a generator that supplies electrical power to a power grid. Exhaust from at least some known gas turbine engines is supplied to a heat recovery steam generator that generates steam for driving a steam turbine. Power generated by the steam turbine also drives an electrical generator that provides additional electrical power to the power grid.
The products of combustion and syngas generated in combustion zone of the gasifier, including gaseous byproducts, slag, soot, char, unreacted carbon, ash, refractory compounds, and inert process materials, arc collected in process water at the bottom of the gasifier. The process water containing the suspended process solids is referred to as black water. For process efficiency, it is desirable to separate the suspended solids from the black water so that the process water can be recycled or used in other processes. Typically, settling and filtering methods are used to remove the suspended solids from the black water with known filtering methods utilizing a precoat material. These methods convert black water to a process water containing less solids, which is referred to as grey water. A portion of this grey water is purged (blown down) to prevent the buildup of dissolved contaminants and fouling or erosive solids in the gasification process. The grey water blow-down needs to be treated for remove the contaminants prior to reuse or disposal to the environment, One of the disposal processes of grey water is deep well injection (DWI). Prior to injection, the grey water is treated to meet required characteristics that prevent harmful effects in the deep well. For example, in one known gasification project, the required characteristics of the grey water for DWI includes less than about 2 mg/L of total suspended solids, less than about 2 microns for the solids particle size, and a pH of about 4 to about 5. A known filtering method is used to remove the suspended solids from the grey water. The known filtering method utilizes a precoat material of diatomaceous earth, aluminum silicates, cellulose, perlite, activated carbon, wood flour, and/or other known commercial blend precoat/bodyfeed products. The use of these known precoat materials adds cost to the gasification process, and some of the known precoat materials, for example diatomaceous earth, have limited availability.